Dosage compensation is a striking example of the interplay between gene-specific regulation and chromosomal architecture. This process has evolved to make X-linked gene expression equivalent in males with one X chromosome and females with two. In species examined at the molecular level, dosage compensation is mediated by sex-specific factors that decorate the X chromosomes. In Drosophila, dosage compensation is achieved, at least in part, through site-specific histone H4 acetylation, modulated by a malespecific, X-specific ribonucleoprotein complex composed of MSL proteins and non-coding roX RNAs. Our focus in the coming grant period will be to understand how chromatin activation is targeted and spread along a chromosome. Our current data suggest that sites of noncoding roX RNA synthesis act as nucleation sites for spreading of MSL complexes in cis. Our experiments will test a model for distribution of MSL complexes in which local spreading in cis from roX genes is balanced with diffusion to additional sites in trans. We will analyze the dynamics of MSL complex establishment and maintenance on the X, and autoregulation of roX RNA by MSL complexes. Our model for spreading of MSL complexes raises interesting parallels with mammalian dosage compensation. In both flies and humans, regulatory molecules are normally restricted in cis to the X chromosome, but if brought to autosomes, can spread on genes never before dosage compensated. The organization of chromatin domains by nucleation sites is likely to be an important general mechanism for regulation of genome function. Thus, dissecting the mechanisms underlying these chromatin-based regulatory processes should provide insight into many important biological problems, including normal and disease states in humans. The superb spatial resolution of polytene chromosomes and the availability of mutants in the protein and RNA spreading components make the MSL complex an excellent model system to determine how changes in chromatin architecture affect gene expression in complex organisms.